Why waste a relatively good BS on flamers? Dedicate a Platoon Command Squad to flamer duty if you want to torch stuff.
Otherwise, it depends on what you're mounted them in, how you're deploying them, and what army you're fighting.
Harker would be good with a Vet squad that has special weapons as opposed to heavy weapons since they could all move and shoot. With Harker in the mix, I think I'd be fielding Grenade launchers against non-MEQ and Plasma for MEQ armies (assuming you're going to use them against infantry).
I don't think opponent's coversaves should be as much of a concern if you have them mounted in a chimera or valk/ven since you will likely have a good mobility to get into a good position before firing.